Stimulation of inositol phospholipid hydrolysis is the initial intracellular response to transmembrane receptor activation by a wide array of extracellular chemical signaling messengers. Inositol phospholipid hydrolysis is stimulated by most extracellular signaling molecules through G protein subunits which activate phospholipase C-beta (PLC-beta) enzymes. Further regulation (e.g. inhibition) of inositol phospholipid hydrolysis may be mediated by alternative factors including covalent modification and proximity to membrane bound substrate. This project's long-term goal is an accurate, rigorous description of PLC-beta3 isoenzyme regulation by kinases, lipid binding domains, and non-G protein intermolecular interactions. The kinetics of phosphorylation-mediated inhibition of PLC-beta3 hydrolytic activity will be quantitated in well-controlled in vitro assays. Phospholipid and membrane binding affinities of putative lipid binding domains within the PLC-beta3 molecule will be assessed to further associate structure with function and regulation of PLC- beta activity. Additionally, the association of PLC-beta3 with putative interacting proteins, other than G proteins or kinases, will be identified and the function of these intermolecular interactions quantitated both in vivo and in vitro. These studies, which aim to detail at a molecular level multiple means of regulation of PLC-beta3 activity beyond the well-characterized G protein activation pathway, will contribute to a greater understanding of the basic mechanisms of signal transduction. A detailed, functional map of PLC-beta enzyme regulation may contribute to the design of novel therapeutic agents that intervene selectively into pathophysiological PLC-beta-mediated processes; hormone and neurotransmitter responses associated with PLC-beta activation include, among many others, smooth muscle contraction, platelet aggregation, hormone secretion, smooth muscle hypertrophy and hyperplasia, neuronal activation, and malignant cell proliferation.